The invention deals with the practical operation of separating liquids from solids by pressing. This operation has many industrial applications including the separation of fibers or solid materials from water or liquids of any kind in the sugar industry, in pulp and paper manufacturing, in chemical manufacturing and in other fields.
For example, in the processing of wood or any other lignocellulosic material for the production of pulp and paper, the raw material is subject to a series of treatments in water suspension or with chemical solutions at relatively low solid-to-liquid ratios or consistencies that require subsequent dewatering steps.
In applications where only slight dewatering and consistency increases are desired, vacuum or pressurized screens or filters are commonly used. In applications where higher consistencies must be reached, as in feeding pressure vessels, digesters or chemical reactors, presses of many designs have been built and operated. These may be classified in the following three main categories:
(1) Roll presses in which the material is pressed between two rolls, perforated or not, such as in paper making, sugar cane crushing and others. PA0 (2) Disk (or cone) presses in which the material is pressed between two perforated disks (or cones) rotating around their respective axes that are set at an angle relative to each other. PA0 (3) Screw presses in which the material is pressed by a specifically designed screw rotating inside a cylindrical or conical perforated barrel.
Presses are particularly useful when substantial dewatering is desired because of the high consistencies they can provide as compared to other dewatering devices. This becomes particularly important as a water and energy saving factor in vapor phase cooking systems, in high consistency washing and bleaching procedures and in other operations. However, despite these obvious advantages, presses are not generally used as effectively as they could be for several reasons, among which are the following:
Roll presses are expensive and quite delicate in their operation since hard foreign bodies, even if small, may damage the roll surfaces, especially if the latter are of the perforated type. PA1 Disk presses are expensive, have small drainage surface and feed ports requiring pre-dewatered material, and the sealing of moving parts against the casing of the machine is difficult and generally inefficient. PA1 Commercially available screw presses, though generally less expensive and of simpler design than other types, are unstable in their operation and difficult to control.
The shortcomings of prior art screw press operation can be explained as follows.
Most materials subjected to dewatering by pressing require increasing pressure increments for every equal increment of liquid removal. For this reason, the pressure follows an exponential curve relative to the volume reduction curve. This relationship means that at the last stages of the compression, corresponding to the higher pressures, a small difference in the compression ratio or a small excess in the material feed rate to a screw press having a fixed configuration may cause a sudden and substantial increment of the pressure that may "jam" the machine, meaning that the material being pressed may become very hard because of excessive dewatering. Consequently, the power needed to drive the machine may exceed that available, or the pressure may reach a level that may damage the machine itself. When a "jam" occurs the press must be stopped and any over-compressed material must be freed before the press can resume operation. On the other hand, if the feed rate or the consistency of the feed slurry momentarily decreases below the designed value, even by a small margin, the press may "slip" meaning that no compression results from the rotation of the screw element because the material in the press rotates with the screw.
The proper operation of such a press requires, therefore, a gradual increase of pressure from the inlet to the outlet. Such a condition is unstable, because a pressure increase at any point, due to any variation of the slurry feed rate or consistency, tends to increase the dewatering, and this, in turn, results in increased friction which generates still further pressure increases. Conversely, a decrease of pressure, resulting from an opposite event, tends to decrease the dewatering, and that generates a further decrease of the internal friction and of the pressure-generating capability of the machine. In the former event the machine will tend to jam and in the latter, to slip.
It is apparent that presently available screw presses are characterized by an unstable operation since any deviations from stable operating conditions tend to become larger rather than be corrected by the natural dynamics of the machine operation. By contrast, the present invention provides a means for stabilizing the operation of screw presses at a desired level of performance, in a manner such that deviations from optimum operating conditions are readily corrected.